**Core Concept**
Ivermectin is a broad-spectrum antiparasitic agent primarily used to treat infections caused by various nematodes (roundworms) and ectoparasites. Its mechanism of action involves binding to glutamate-gated chloride channels in invertebrate nerve and muscle cells, leading to hyperpolarization and paralysis of the parasite.

**Why the Correct Answer is Right**
Ivermectin is not indicated for malaria treatment because it targets a different class of parasites altogether. Malaria is caused by Plasmodium species, which are protozoa, not nematodes. Ivermectin is ineffective against Plasmodium due to its specific mechanism of action, which is directed against invertebrate parasites. In contrast, ivermectin is highly effective against nematodes, such as Ascaris, and ectoparasites.

**Why Each Wrong Option is Incorrect**
**Option A:** Ivermectin is indeed used to treat Ascaris infections, making this option incorrect. Ascaris lumbricoides is a nematode that causes ascariasis, which is effectively treated with ivermectin.

**Option B:** Ivermectin is also used to treat filariasis, caused by filarial nematodes. This option is incorrect because ivermectin is a well-established treatment for filariasis.

**Option D:** Ivermectin is used to treat onchocerciasis (river blindness), caused by Onchocerca volvulus, a filarial nematode. This option is incorrect because ivermectin is a standard treatment for onchocerciasis.

**Clinical Pearl / High-Yield Fact**
When treating parasitic infections, it is essential to identify the correct parasite species to choose the most effective treatment. Ivermectin is a valuable agent in the treatment of nematode and ectoparasite infections, but it is not suitable for protozoal infections like malaria.

✓ Correct Answer: C. Malaria